Indeed, there is known a system of bar coding such as the one referred to as "CMC 7" which is used for coding data on bank cheques, such as, for example, their number.
The fixed bar code is read by magnetic detection of the bars (after prior magnetization) and for decoding, the spacing between the bars or the spacing between the upstream and downstream points of these bars is interpreted.
The invention provides for a magnetic binary code which can be modified at will (as in the case of ordinary magnetic tracks) for superposition on the well-known fixed code.
The invention provides a method of superposing a modifiable magnetic code on a fixed code constituted by a succession of magnetized bars separated from one another by spaces, wherein the bars are magnetized in two different directions by a write head for the modifiable magnetic code.
According to a first means of performing the method each bar of the succession of bars is magnetized in only one of said two directions, one of the two magnetization directions representing one of the two values of a binary variable and the other direction representing the other value of the variable.
According to a second means of performing the invention each bar of the succession of bars is devided into two zones, one of which is magnetized in one of said two directions and the other of which is magnetized in the other of said two directions, the order in which these two magnetization directions succede one another on a single bar determining the binary value of the bar the modifiable magnetic code.
According to a third means of performing the method, one of the two binary values of the modifiable magnetic code is represented by a bar magnetized in a single direction, while the other binary value is represented by a bar divided into two zones magnetized in opposite directions from each other.
The invention also provides apparatus for superposing a modifiable magnetic code on a fixed code according to the method, the apparatus including first and second magnetic heads and means for causing relative movement of the bar code supporting medium and the heads in such a manner that the bar code moves firstly past the first head and secondly past the second head, the first head including means responsive to the bar code to store an image thereof and the second head including firstly means responsive to the bar code to cause the stored image there of to be read in synchronism with the passage of the bar code past the second head and secondly means responsive to the synchronously read image to write a modifiable message on the bars of the bar code.
The invention also provides apparatus for superposing a modifiable magnetic code on a fixed bar code according to the third means of performing the method, the apparatus including a magnetic head which includes a read winding and a write winding and means for allowing writing only on the second half of any bar.
According to a preferred embodiment said means include a generator which emits a signal when a bar begins to pass under the air gap of the magnetic head which signal starts a timer unit whose output is connected to a write generator connected to the write winding.
Devices embodying the invention are described greater detail by way of example with reference to the accompanying drawings.